The present invention relates to mixed irradiation using proton beams and X rays, and in particular, to a method of supporting treatment planning for effectively applying a certain dose of radiation on a focus (malignant tissue) while suppressing normal tissue's exposure to radiation.
In radiotherapy that uses radiation for treatment, it is desired that normal tissues' exposure to radiation is suppressed as much as possible while applying a sufficient therapeutic dose of radiation on a focus such as cancer. Accordingly, treatment planning is required before treatment, in order to judge and determine intensity and a direction of radiation to emit, utilizing image data obtained by a video diagnostic apparatus such as an X-ray CT (Computerized Tomography) unit, and based on results of simulation of a dose distribution or the like.
Generally, such treatment planning is carried out by software that runs on a computer system. First, in treatment planning, image data are used in order to set three-dimensional areas of a focus and noteworthy normal tissues around the focus, and their coordinates are stored into a memory. Next, a three-dimensional dose distribution inside a human body is calculated according to a certain physical model and using the image data, based on an irradiation range (called a radiation field), which is determined according to the size of the focus, and a tentatively-determined direction and intensity of irradiation.
Thus-obtained results are evaluated by various evaluation techniques. As such techniques, for example, may be mentioned DVH (Dose Volume Histogram), i.e., a graph expressing a relation between a dose and a tissue volume having a dose value thereof, an isodose map (isodose contour map) in which a dose distribution is superposed onto a body tomographic image, and three-dimensional displaying in which a dose distribution is superposed as three-dimensional data onto body tissues and is displayed semi-transparently and stereoscopically. When it is judged by these techniques that the dose distribution is desirable one, then, the tentatively-determined direction and intensity are employed for treatment. If not, the direction and intensity of irradiation are determined anew, and the dose distribution is calculated again to evaluate the results. Usually, in treatment planning, such operations are repeated to determine the direction and intensity of irradiation to be employed for treatment.
On the other hand, recently, proton treatment facilities have been constructed in various places of the world. In these facilities, a proton beam generated by an accelerator (particle accelerator) is used for treatment, and treatment by a proton beam is started in parallel with the conventional X-ray treatment. As an example of a proton treatment facility, may be mentioned Proton Treatment Center in Loma Linda University Medical Center in USA.
A proton beam used in proton beam treatment has a characteristic called “Bragg Peak”. Namely, when a proton beam is irradiated onto a material, the dose increases rapidly to become the maximum at a certain depth from the surface, and decreases rapidly to zero in a deeper area than that depth. This location at which the dose becomes the maximum is called Bragg Peak. By controlling the location of Bragg Peak, it is possible to apply a large dose to a focus, generally conforming to the shape of the focus, and scarcely applying a dose to the back of the focus.
In contrast, an X-ray used in X-ray treatment has a characteristic that the dose declines exponentially according to the depth from a surface. Thus, the X-ray does not have Bragg Peak. Accordingly, in a usually-employed method, in order to suppress exposure of normal tissues as much as possible, radiation is applied from a plurality of directions toward a focus, in order to apply a large dose to the focus locally. For generating an X-ray, so large scale an accelerator is not required as in the case of a proton beam. Thus, conventionally, an X-ray has been widely used in medical facilities. X-ray treatment is described in Takuro Arimoto, et al.: “Small volume Multiple non-coplanar Arc Radiotherapy (Smart) for tumors of the lung, head&neck and abdominopelvic region”, CAR' 98 Proceedings, pp. 257 –261, for example.